How to determine which vertex groups a vertex belongs to?
You can press shiftG (Select Similar) > Vertex Groups.



Also, a more precise method, in your 3D view, if you select the vertex and press N, you'll have access to the 3D view properties panel, if you press the Item tab, you'll see under Vertex Weight what group the vertices are part of. Then you can select them in the Properties panel > Data > Vertex Groups.